Medical science has made great strides in enabling the profoundly deaf to hear. Such individuals can now hear with the aid of a specialized neural stimulation device referred to as a “cochlear stimulator.”
An example of an implantable cochlear device that implements the above process has two main components: (a) a wearable or external system and (b) an implantable system. The external (wearable) system can include a speech processor coupled to a headpiece. The speech processor can include a battery or equivalent power source and further include electronic circuitry such as a microprocessor that converts sound waves to electrical signals and processes these signals in accordance with a desired speech processing strategy. The headpiece, which can be adapted to be worn by a patient behind the ear, can include a microphone for capturing ambient sound waves and converting them into electrical signals and, further, include an antenna or coil for transmitting the processed signals through the skin to the implantable system.
The implantable system, referred to hereinafter as an “implantable cochlear stimulator” (“ICS”), contains no power source, but instead receives its operating power from the external system which contains the battery. The ICS outputs a multiplicity of current stimulation channels, each channel connected to at least one electrode contact within the cochlea. Typically, however, each stimulation channel is uniquely connected to only one stimulating electrode, although the return connection of one channel may be connected uniquely to one return electrode contact or, alternatively, more than one return connection may be connected to a common, indifferent electrode such as the case of the ICS. Electronic circuitry can also be included in the ICS that permits the sign (polarity) and magnitude of the output current of each of the stimulation channels to be programmably specified at short-duration, predetermined intervals. An example of an implantable system which receives power from an external power source is provided in U.S. Pat. No. 5,603,726 which is incorporated herein by reference.
In operation, the microphone converts ambient sound waves into electrical signals which are processed by the signal processor. The processed signals are passed transcutaneously via a pair of electromagnetically coupled transmitter/receiver coils. The signals are further processed and transformed within the implantable system to generate complex stimulation waveforms at the times and durations specified and these waveforms are passed to the intra-cochlear electrodes which deliver electrical stimulation currents to the auditory nerves. As a result, the individual perceives an auditory sensation.
Detection of the speech processor data signals within the ICS electronic circuitry is necessary for a number of functions. A conventional data detection function in cochlear implant devices is performed by measuring the average voltage developed across a current sensing resistor which is placed in the RF power rectification circuit. This detection circuit provides a signal having good fidelity, but disadvantageously, can also reduce the voltage and power available to the stimulation channels, since the detection circuit is directly inserted within the rectifier. Conserving device power is critically important in a battery-powered, implantable, cochlear stimulation system, since it is inconvenient for the user to frequently recharge or replace batteries. Furthermore, a rechargeable battery can only be recharged a finite number of times before useful end of life is reached. As such, frequent battery recharging hastens the end of useful life, at which point, the battery must be discarded.
Conserving battery power is even more important with advanced cochlear implant devices capable of processing complex, multi-channel signals because such devices consume battery power more quickly than older systems having fewer channels and simpler processing. In addition, a two-part cochlear implant system, comprised of an external processing unit and an implantable unit, commonly employs a pair of transmitter/receiver coils which are electromagnetically coupled. Such a coupling can be relatively inefficient and can dissipate a substantial amount of power. It is important, therefore, that every circuit in the device, including any detection circuit, be optimized for the most efficient operation.
A conventional data detection circuit presents another disadvantage in that the signal voltages developed are below the lowest voltage available in the cochlear implant and, consequently, the weak signals obtained may be difficult to process. It would be desirable to have a detection system which can provide stronger voltage signals for easier processing, yet do so efficiently.
Thus, there is a need for a data detection circuit to be used in a cochlear implant device that conserves available battery power, while providing detected signal voltages which can be easily processed.